Cashflow forecasting

In additive model, I did everything same. Only the difference is in taking the calculation of change in trend to find forecast.
I used the first and last trend of the information and divided by number of trend (less 1).
In example it is calculating trend change by trend of first quarter of Year x6 till first quarter of year x7 and divided by 4.
Why it is taking on quarter bases. In examples before we did not took change like this. A question after this, on page 379, under heading of “Forecast Sales”, we are taking increase by taking difference in last and first trend and divide it by the number of trends. Why the author is changing method again and again?
(Reference: MA2 BPP book, Page 378-379, example 9.1)